TransactionParticipantServiceImpl.findGroupIdByName(): add unit tests

[0pdd]

The puzzle 857-cbb24247 from #857 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/service/TransactionParticipantServiceImpl.java

Line 99 in 95faa9a

// @todo #857 TransactionParticipantServiceImpl.findGroupIdByName(): add unit tests

The puzzle was created by Slava Semushin on 15-Jul-18.